Electrode for p-type SiC

ABSTRACT

An object of this invention is to provide an electrode for p-type SiC which can provide improved surface morphology and less thermal damage for a semiconductor crystal layer due to formation of an electrode. In this invention, a p-type electrode is manufactured to contain at least one selected from the group consisting of nickel (Ni), cobalt (Co), palladium (Pd) and platinum (Pt).

TECHNICAL FIELD

This invention relates to a silicon carbide (SiC) device, and moreparticularly to an electrode for p-type SiC, which is employed in an SiCdevice.

BACKGROUND ART

It has been expected that silicon carbide (SiC) is applied to a highfrequency electric power device, a high temperature device and anoptoelectronics device, and its investigation for actual use is nowunderway. The p-electrode for the SiC device, i.e. an ohmic electrodefor p-type SiC is generally formed of TiAl composed of titanium (Ti) andaluminum (Al) (see JP-A-5-13812).

Generally, in many cases, the device needs an ohmic electrode. However,in the device using compound semiconductor, generally, an ohmic contactbetween a semiconductor layer and an electrode cannot be obtainedwithout being subjected to heat treatment. Only provision of a metalliclayer exhibits a Schottky barrier. The ohmic contact formed by the heattreatment provides greatly different contact resistances according to asemiconductor material, an electrode material, a heat treatmenttemperature, a heat treatment time, etc.

TiAl, which is presently expected as a material for a low resistanceohmic contact, can provides very low resistance. However, in order torealize such low resistance, heat treatment at a high temperature for along time as well as a large amount of Al is required. This leads to theproblems of deterioration of a device function, a device life, etc. dueto deterioration of surface morphology and thermal damage for asemiconductor crystal layer.

This invention has been accomplished in order to attain the aboveproblems, and an object of this invention is to provide an electrode forp-type SiC which can provide improved surface morphology and lessthermal damage for a semiconductor crystal layer due to formation of anelectrode.

DISCLOSURE OF THE INVENTION

The inventors of this application devoted theirselves to investigationin order to, and accomplished the following inventions.

An electrode for p-type SiC contains a first electrode material of atleast one selected from the group consisting of nickel (Ni), cobalt(Co), palladium (Pd) and platinum (Pt).

The electrode for p-type SiC having the above configuration can providean ohmic property and improved flatness of the electrode surface by theheat treatment at a lower temperature than before. The above electrodefor p-type SiC provides the ohmic property through the heat treatment ina wide range, for example by that for a short time at a hightemperature. This reduces the thermal affect on the semiconductorcrystal layer due to the formation of the electrode. Thus, by using theelectrode for p-type SiC according to this invention, an SiC device withan excellent device performance can be manufactured.

In this invention, the electrode for p-type SiC refers to the electrodeformed on a p-type SiC semiconductor layer. The type of the p-type SiCsemiconductor to which the electrode for p-type SiC according to thisinvention is applied should not be particularly limited, but includes 6Htype, 15R type, 21R type, 3C type, etc. as well as the 4H type which isadopted in the embodiment described later. The kind of the device towhich the electrode for p-type SiC according to this invention isapplied should not be particularly limited, but may be various deviceswhich are employed in a high frequency power device, a high temperaturedevice, an optoelectronics device, etc.

As the first electrode material, nickel or cobalt is particularlypreferable because nickel or cobalt reacts with Si at a relatively lowtemperature.

The electrode for p-type SiC according to this invention preferablyfurther contains the second electrode material of aluminum (Al) inaddition to the first electrode material. The containing of Al reducesthe contact resistivity and provides the electrode for p-type SiC withan excellent ohmic property.

The electrode for p-type SiC according to this invention preferablyfurther contains the third electrode material of titanium (Ti). Namely,the electrode preferably contains the first electrode material of e.g.nickel (Ni), second electrode material of aluminum (Al), and thirdelectrode material of titanium (Ti). The containing of the thirdelectrode material of titanium (Ti) further reduces the contactresistivity.

The electrode for p-type SiC according to this invention preferablyincludes a layer of the first electrode material (hereinafter referredto as a first electrode material layer). More preferably, the firstelectrode material layer is formed in contact with the p-type SiCsemiconductor layer. For example, by forming the first electrodematerial layer on the p-type SiC semiconductor layer, subsequentlystacking the layer of the other electrode material thereon andheat-treating these layers, the electrode for p-type SiC according tothis invention can be manufactured.

In the case where the electrode for p-type SiC contains the secondelectrode material as well as the first electrode material, theelectrode for p-type SiC according to this invention preferably includesthe first electrode material layer and a layer of the second electrodematerial (hereinafter referred to as a second electrode material layer).In other words, in the manufacturing process, the first electrodematerial layer and the second electrode material layer are preferablyformed. The order of stacking the first material layer and the secondmaterial layer should not be particularly limited, but the firstelectrode material layer and the second electrode material layer arepreferably stacked in order from the side of the p-type SiCsemiconductor layer. Like the above case, the first electrode materiallayer is preferably formed in contact with the p-type semiconductorlayer. A layer of the other material may be located between the firstelectrode material layer and the second electrode material layer. Aplurality of layers of the first electrode material layer and/or thesecond electrode material layer may be formed. For example, the firstelectrode material layer, the second electrode material layer and thefirst electrode material layer may be stacked in order from the side ofthe p-type SiC semiconductor layer to constitute the electrode forp-type SiC according to this invention.

In the case where the electrode for p-type SiC contains the firstelectrode material, second electrode material and third electrodematerial, the electrode for p-type SiC according to this inventionpreferably includes the first electrode material layer, second electrodematerial layer and a layer of the third electrode material (hereinafterreferred to as a third electrode material layer). In other words, in themanufacturing process, the first electrode material layer, secondelectrode material layer and third electrode material layer arepreferably formed. The order of stacking these layers should not beparticularly limited, but the first electrode material layer, thirdelectrode material layer and second electrode material layer arepreferably stacked in order from the side of the p-type SiCsemiconductor layer. Like the above cases, the first electrode materiallayer is preferably formed in contact with the p-type semiconductorlayer. A layer of the other material may be located between the firstelectrode material layer and the third electrode material layer, and/orbetween the third electrode material layer and the second electrodematerial layer. A plurality of layers of the first electrode materiallayer, third electrode material layer and/or the second electrodematerial layer may be formed.

The method of forming the first, the second and the third electrodelayer should not be limited, but can be implemented by molecular beamepitaxy (MBE), sputtering, resistive heating, etc.

The electrode for p-type SiC according to this invention is manufacturedby stacking the above electrode material layer (and other layers) on thep-type SiC semiconductor layer and thereafter heat-treating theselayers. Prior to forming the electrode material layer, the p-type SiCsemiconductor is preferably washed (e.g. chemically washed). Thisintends to stack the electrode materials in a desired state. Theheat-treatment for the p-type SiC is carried out in order to form theohmic contact between the p-type SiC semiconductor layer and theelectrode for p-type SiC according to this invention. The heatingtemperature and the heating time are adjusted as required to provide animproved ohmic contact. The heating temperature may be e.g. 400°C.-1100° C., preferably 600° C.-900° C., more preferably 700° C.-850° C.and most preferably about 800° C. On the other hand, the heating timemay be e.g. 2 min.-100 min., preferably 2 min.-50 min. and morepreferably 5 min.-30 min. Incidentally, the heating is preferablycarried out in a vacuum state. The heating can be carried out in anatmosphere of inert gas. The inert gas may be nitrogen gas, helium gas,argon gas, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the result of Experiment 1, which exhibitsdependency of the contact resistivity (ρc) after the heat treatment at1000° C. for 2 minutes on the Al concentration in NiAl.

FIG. 2 is a graph showing the result of Experiment 2, which exhibits thecurrent-voltage (I-V) characteristic of the CoAl/SiC contact after theheat treatment at 800° C. for 10 minutes. In FIG. 2, symbols arereferred to as no intermediate layer (◯), Al 10 nm (□), Al 40 nm (⋄) andAl 120 nm (Δ).

FIG. 3 is a graph showing the result of Experiment 3, which exhibits thecurrent-voltage (I-V) characteristics of the respective samples (Ti/Al,Ni/Al, Ni/Ti/Al) after the heat treatment at 800° C. for 10 minutes.

-   ▪: Ni (25 nm)/Ti(50 nm)/Al(300 nm), 800° C., 10 min,-   ▴: Ni (44 nm)/Al(53 nm), 800° C., 10 min,-   ●: Ti (50 nm)/Al(300 nm) 800° C., 2 min.

FIG. 4 is a graph showing the result of Experiment 4, which exhibitschanges in the contact resistivity (ρc) when differences in the filmthickness of a Ni layer and Ti layer are changed in a sample having alaminated structure of Ni/Ti/Al.

-   Δ: Ni(8 nm)/Ti(50 nm)/Al(300 nm)-   ⋄: Ni(15 nm)/Ti(50 nm)/Al(300 nm)-   ♦: Ni(25 nm)/Ti(50 nm)/Al(300 nm) (the above samples have Al    concentration of 3.0×10¹⁸ cm⁻³)-   ●: Ni(8 nm)/Ti(50 nm)/Al(300 nm)-   ◯: Ni(15 nm)/Ti(50 nm)/Al(300 nm) (the above samples have Al    concentration of 8.1×10¹⁸ cm⁻³)

FIG. 5 is a schematic view of an SiC device 1 according to an embodimentof this invention.

BEST MODE FOR CARRYING OUT THE INVENTION

An explanation will be given of various experiments according to thisinvention.

Experiment 1

In this experiment, the electric characteristic and surface flatness ofan NiAl system ohmic contact material for a p-type 4H-SiC was examined.

As a substrate, a p-type 4H-SiC(0001) epitaxial wafer (Al concentration:6.4-9.0×10¹⁸ cm⁻³), which is commercially available was adopted. Afterthe substrate had been chemically washed, a thermal oxide film having athickness of 10 nm was deposited thereon, and a circular TLM pattern wasmade by photolithography. After the oxide film had been removed usingdiluted hydrofluoric acid, Ni and Al were deposited by techniques ofelectron beam and resistive heating. Contact materials were made withtheir different thicknesses. After an electrode pattern was formed by alift-off step, heat treatment was carried out within a super high vacuumchamber at 800° C. to 1000°. The resistivity and surface crystallinityof each sample was evaluated by circular TLM, X-ray diffraction (XRD),Rutherford backscattering spectrometry (RBS) and observation by anoptical microscope.

The NiAl contact material exhibited the ohmic property even after theheat treatment at 800° C. Namely, it was revealed that Ni adopted as oneof electric materials provides the ohmic property in also the heattreatment at a lower temperature than before. FIG. 1 shows thedependency of the contact resistivity (ρc) after the heat treatment at1000° C. for 2 minutes on the Al concentration in NiAl.

As seen from FIG. 1, the NiAl contact material provides the contactresistivity ρc on the order of 10⁻⁴ Ωcm² at the concentration (40 at %or lower) lower than that of a TiAl contact material, and provides ρc of9×10⁻⁵ Ωcm² at the Al concentration of 80 at %. With an increase in theAl concentration, the contact resistivity decreases. Thus, it wasconfirmed that Al plays an important part in the reduction of thecontact resistivity. Further, after the heat treatment at 1000° C. also,NiAl provided a flatter surface than in TiAl.

Experiment 2

In this experiment, the heat treatment temperature and electrode surfaceflatness of a CoAl ohmic contact material for a p-type 4H-SiC wereexamined.

As a substrate, a p-type 4H-SiC(0001) epitaxial wafer (Al concentration:1.0×10¹⁹ cm⁻³), which is commercially available was adopted. After thesubstrate had been chemically washed, a thermal oxide film having athickness of 10 nm was deposited thereon, and a circular TLM pattern wasmade by photolithography. After the oxide film had been removed usingdiluted hydrofluoric acid, metallic layers of Co and Al were depositedby techniques of electron gun deposition and resistive heating. Aplurality of samples with different thicknesses of the intermediatelayer in a Co/Al/Co deposition structure were made to have the entirefilm thickness of 180 nm. After an electrode pattern was formed by alift-off step, heat treatment was carried out within a super high vacuumchamber for between 10 minutes at 800° C. and 2 minutes 1000° C. Theresistivity and surface crystallinity of each sample was evaluated bycircular TLM, X-ray diffraction (XRD), Rutherford backscatteringspectrometry (RBS) and observation by optical microscope.

FIG. 2 shows the current-voltage (I-V) characteristic of the CoAl/SiCcontact after the heat treatment at 800° C. for 10 minutes. As seen fromthe graph of FIG. 2, it was confirmed that the samples having thin Alintermediate layers of 10 nm and 40 nm, respectively, provide animproved ohmic property. The heat treatment further carried out at 1000°C. for 2 minutes reduced the contact resistivity to 4×10⁻⁴ Ωcm². TheCoAl contact after the heat treatment at 1000° C. also gave a very flatsurface as compared with the TiAl contact. It was revealed that the Cocontact material which does not includes an Al layer cannot give theohmic property after the heat treatment at 1000° C. and hence Al playsan important part in the reduction of the contact resistivity.

Experiment 3

In this experiment, the heat treatment temperature and electrode surfaceflatness when Ni is employed in an ohmic contact material for a p-type4H-SiC were examined.

As a substrate, a p-type 4H-SiC(0001) epi-wafer (Al concentration:3.0-8.1×10¹⁸ cm⁻³), which is commercially available was adopted. Afterthe substrate had been chemically washed, a thermal oxide film having athickness of 10 nm was deposited thereon, and a circular TLM pattern wasmade by photolithography. After the oxide film had been removed usingdiluted hydrofluoric acid, metallic layers of Ni and Ti were depositedby the technique of electron beam, and an Al metallic layer wasdeposited by resistive heating. The laminated structure was made foreach sample of Ti/Al (Ti layer and Al layer are stacked in order fromthe side of SiC. The same shall apply hereinafter), Ni/Al and Ni/Ti/Al.For Ni/Ti/Al, a plurality of samples with different thicknesses of theNi layer and Ti layer were made. Incidentally, the vacuum degree duringthe deposition was set at 1×10⁻⁶ Torr. After an electrode pattern wasformed by a lift-off step, heat treatment was carried out within a superhigh vacuum chamber for 5 to 30 minutes at 800° C. The resistivity andsurface crystallinity of each sample was evaluated by circular TLM,X-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS)and observation by an optical microscope.

FIG. 3 shows the current-voltage (I-V) characteristics of the respectivesamples (Ti/Al, Ni/Al, Ni/Ti/Al) after the heat treatment at 800° C. for10 minutes.

-   ▪: Ni (25 nm)/Ti(50 nm)/Al(300 nm), 800° C., 10 min,-   ▴: Ni (44 nm)/Al(53 nm), 800° C., 10 min,-   ●: Ti (50 nm)/Al(300 nm) 800° C., 2 min.

As seen from FIG. 3, for the sample of Ti/Al, the heat treatment at 800°C. did not gave the ohmic property, whereas for the samples (Ni/Al,Ni/Ti/Al) including the Ni layer, the heat treatment at 800° C. gave theohmic property. In other words, it was confirmed that provision of theNi layer permits the ohmic contact to be formed by the heat treatment ata lower temperature than before. Further, the surface of each of NiAland Ni/Ti/Al after the heat treatment was very flat.

FIG. 4 shows the relationship between the film thickness of a Ni layerand Ti layer and the contact resistivity (ρc) in a sample having alaminated structure of Ni/Ti/Al.

-   Δ: Ni(8 nm)/Ti(50 nm)/Al(300 nm)-   ⋄: Ni(15 nm)/Ti(50 nm)/Al(300 nm)-   ▴: Ni(25 nm)/Ti(50 nm)/Al(300 nm) (the above samples have Al    concentration of 3.0×10¹⁸ cm⁻³)-   ●: Ni(8 nm)/Ti(50 nm)/Al(300 nm)-   ◯: Ni(15 nm)/Ti(50 nm)/Al(300 nm) (the above samples have Al    concentration of 8.1×10¹⁸ cm⁻³)

It was revealed that an increase in the thickness of the Ni layerreduces the resistivity, i.e. the resistivity depends on the thicknessof the Ni film. Incidentally, the condition of Ni(25 nm)/Ti(50nm)/Al(300 nm) realized ρc=6.64×10⁻⁵ Ωcm².

An explanation will be given of an embodiment of this invention.

FIG. 5 is a schematic view of the structure of an SiC device 1 accordingto an embodiment of this invention.

The SiC device can be manufactured by the following process.

First, an n-type SiC substrate 10 is placed in a vapor-phase growingapparatus chamber. Using hydrogen gas as a carrier gas, monosilane(SiH₄) and propane (C₃H₈), which are a raw gas, andtrimethylaluminum((CH₃)Al),which is an impurity gas, are supplied intothe chamber to form a p-type SiC layer 11 having a thickness of about 5μm at a growing temperature at about 1400° C. Incidentally, the p-typeSiC layer 11 can be also formed by known molecular beam epitaxy (MBE),halide vapor phase epitaxy (HVPE), sputtering, ion-plating, electronshower, etc.

The substrate 10 is subjected to sacrificial oxidation in an atmosphereof O₂ for 60 minutes at 1150° C. to deposit an SiO₂ film 12 having about10 nm on the surface of the p-type SiC layer 11. After electrodepatterning by photolithography is performed, a part of the SiO₂ film isremoved using diluted hydroflouric acid. Subsequently, an Ni layer 21having a thickness of about 25 nm is formed by the electron beam method.Likewise, a Ti layer 22 is formed by the electron beam method and an Allayer 23 is formed by the resistive heating. Thereafter, an electrodepattern is formed by a lift-off step. Through the process describedabove, a p-type electrode 20 composed of Ni, Ti, and Al which have beenstacked in order is manufactured as shown in FIG. 5.

Next, in order to make an ohmic contact between the p-type SiC layer 11and the p-type electrode 20, heat treatment is carried out in the superhigh vacuum chamber for 10 minutes at 800° C.

Subsequently, an n-type electrode 30 which is composed of vanadium (V)and aluminum (Al) is formed on the surface of the n-type SiC substrate10 by vapor deposition. Upon completion of the process described above,the step of separating the chip is carried out using a scriber toprovide the SiC device 1.

This invention has been hitherto described in detail and with referenceto a specific embodiment. However, it is apparent to those skilled inthe art that various changes or modifications can be made withoutdeparting from the spirit and scope of this invention.

This application is based on Japanese Patent Application No. 2001-270771filed on Sep. 6, 2001 and its contents are incorporated hereby byreference.

INDUSTRIAL APPLICABILITY

This invention should not be limited to the embodiment described above.This invention includes various embodiments within a range which doesnot deviate from the scope of claim and can be easily anticipated bythose skilled in the art. Further, it is needless to say that thisinvention can be applied to other semiconductor devices using SiC, e.g.III-group nitride compound semiconductor on SiC.

1. An electrode for p-type SiC, comprising: a first electrode materialof at least one member selected from the group consisting of nickel(Ni), cobalt (Co), palladium (Pd) and platinum (Pt); a second electrodematerial of aluminum (Al), said second electrode material comprising athickness of 50 nm; and a third electrode material of titanium (Ti),said third electrode material comprising a thickness of 300 nm, whereina thickness of said first electrode material is selected for providing adesired contact resistivity for said electrode.
 2. An electrode forp-type SiC according to claim 1, wherein said first electrode materialcomprises one of Ni and Co.
 3. An electrode for p-type SiC arcording toclaim 1, wherein said first electrode material is formed in contact witha p-type SiC layer.
 4. An SiC device, wherein the electrode for p-typeSiC described in claim 1 is formed on a p-type SiC layer.
 5. Theelectrode for p-type SiC according to claim 1, wherein a heatingtemperature for said p-type SiC to form an ohmic contact comprises atemperature of 600° C. to 900° C.
 6. The electrode for p-type SiCaccording to claim 5, wherein said heating temperature comprises atemperature of 700 ° C. to 850° C.
 7. A SiC device comprising: a p-typeSiC layer; an electrode comprising a first electrode material selectedfrom the group consisting of nickel (Ni), cobalt (Ca), palladium (Pd),and platinum (Pt); and a SiO₂ film deposited on and in contact with saidfirst surface of said p-type SiC layer, wherein a thickness of saidfirst electrode material is selected for providing a desired contactresistivity for said electrode.
 8. The SiC device according to claim 7,further comprising: a second electrode material of aluminum (Al) formedon said first electrode material.
 9. The SiC device according to claim8, further comprising: a third electrode material of titanium (Ti)formed on said second electrode material.
 10. The SiC device accordingto claim 7, wherein a heating temperature for said p-type SiC layer toform an ohmic contact comprises a temperature of 600° C. to 900° C. 11.The SiC device according to claim 10, wherein said heating temperaturecomprises a temperature of 700° C. to 850° C.